Presbycusis
The morbidity rate for various degeneration diseases increases along with human longevity. The common degeneration diseases are the degeneration of bones and sensory functions, wherein the degeneration of auditory functions is one of the most common sensory degeneration diseases in the elderly.
Presbycusis is also named as age-related hearing loss (ARHL), which means that the level of hearing deterioration of the two ears worsens as patients get older. Basically, ARHL is further divided into neural hearing loss and sensory hearing loss, wherein neural hearing loss is caused by auditory (acoustic) nerve deterioration, and sensory hearing loss is caused by cochlear deterioration. However, both losses are clinically difficult to distinguish from each other. Thus, both losses are collectively nominated as sensorineural hearing loss. The main causes of the hearing loss are considered as senescence, oxidation damage, mitochondrial damage or environmental pressure. This hearing damage usually is prevalent in people more than 50 years old, and the earliest symptom is hearing loss at high frequencies. That is, compared to the vowels in a language, it is increasingly difficult to hear consonants (the high-frequency portion of a language). The symptoms mentioned above are particularly significant in a noisy environment. A patient's central nervous system will slow down the processing rate of the auditory messages, so that the patients need to spend more time to decode the received auditory messages. Clinically, the pathological characteristics of the disease are an increased hearing threshold shift and decreased speech comprehension. These symptoms affect the patients' capacity for voice reception and language recognition, causing a significant communication disability, that affects socializing, expressing emotions, and the quality of life.
According to the pathological sites for presbycusis, H. F. Schuknecht divided presbycusis into four types in the report “Cochlear pathology in presbycusis” (Schuknecht, H. F. and Gacek, M. R. Cochlear pathology in presbycusis. Ann. Otol. Rhinol. Laryngol. 1993. 102 (1 Pt 2): 1-16):
1. Sensory presbycusis: The numbers of sensory cells and supporting cells in the corti become fewer or their functions deteriorate.
2. Neural presbycusis: This is caused by the decreased number of spiral ganglion cells or the neuronal degeneration, thereby affecting the transmission of hearing messages.
3. Metabolic/strial presbycusis: The degeneration and atrophy of the stria vascularis result in insufficient bloodstream for transporting nutrients.
4. Mechanical/cochlear presbycusis: This is caused by basement membrane degeneration, including the thickened width, the reduced elasticity or the hardening (due to calcification) of the basement membrane.
Hericium erinaceus 
According to the description in Medicinal Fungi of China, “H. erinaceus tastes sweet, is neutral and tonic, has benefits for five viscera and digestion, and has excellent effects on dyspepsia, neurodegeneration, duodenal ulcers and gastric ulcers” (Mao, X.-L. Chinese edible and pharmaceutical large fungi. Microbiology China, 1989. 16(5):290-297). Therefore, it is known that H. erinaceus, a pharmaceutical and edible fungus, has a history of disease treatment in ancient medicine. H. erinaceus is classified in the kingdom Fungi, the phylum Eumycota, the subphylum Basidiomycotina, the class Basidiomycetes, the order Aphyllophorales, the family Hydnaceae, the subfamily Hericioideae, and the genus Hericium. H. erinaceus's fruiting body has a soft and spherical appearance with long rough protuberances. H. erinaceus is white when fresh and turns tawny after being dried. The H. erinaceus fruiting body or mycelia extract contains saccharides (Wang et al., Kaohsiung J. Med. Sci., 2001, 17(9):461-467; Yang et al., Biosci. Biotechnol. Biochem., 2003, 67(6):1292-1298), erinacines (Saito et al., J. Antibiot., 1998, 51(11):983-990; Kenmoku et al., Biosci. Biotechnol. Biochem., 2002, 66(3):571-575; Kenmoku et al., Biosci. Biotechnol. Biochem., 2004, 68(8):1786-1789; Watanabe et al., Org. Lett., 2007, 9(2):359-362; Watanabe and Nakada, J. Am. Chem. Soc., 2008, 130(4):1150-1151; Lee et al., Int. J. Mol. Sci., 2014, 15(9):15073-15089; Li et al., Food Chem. Toxicol., 2014, 70:61-67), dilinoleoyl-phosphatidylethanolamine (DLPE) (Nagai et al., J. Nutr. Biochem., 2006, 17(8):525-530), amino acids, proteins, and minor elements (Jia et al., Carbohydr. Res., 2004, 339(16):2667-2671). In the literature, it is common knowledge that polysaccharides of H. erinaceus have effects on immunoregulation, blood lipid reduction, blood sugar reduction, gastric inflammation inhibition, or stomach cancer prevention. However, there have been no literature showing that H. erinaceus has the effect of preventing hearing deterioration.